The present invention relates to an image data compressing method in use with an image processor which can receive and output multi-valued image data.
In a digital copying machine, for example, an image input unit reads image data on an original document, and converts the data into a digital image signal. The digital image signal is temporarily stored in a memory. Various types of image processing are applied to the digital image data. Finally, an image output unit outputs the processed image data onto a sheet of record paper.
In the image data processing system of the type in which image data is processed after being converted to digital image data, the quantity of data to be handled is remarkably increased as a definition of the digital image becomes high and the images to be handled become color images. To store such a large quantity of data, a large memory capacity is required for the memory. The memory of the large memory capacity is expensive, and consequently increases the cost to manufacture.
To cope with the problem, an attempt to use the data compressing techniques has been made.
A discrete cosine transform coding method (referred to as a DCT method), as one of the data compressing techniques, will be incorporated into the international standards. Preparatory work for the international standards is currently progressing under the cooperation of ISO and CCITT. The DCT method has been known as a method capable of compressing the image data of a natural image at a high efficiency. As also known, the method has a shortcoming that the compression ratio is highly responsive to and readily varies with the high frequency components contained in the image data. The variation of the compression ratio must be made to converge to a predetermined value. The cost (computational quantity and time) to realize this is large.
Another coding method is proposed in the following literatures (1) and (2). In the coding method, an average energy of the whole image and a distribution of local energies of the image are measured before the image data is coded. The coding parameters are controlled so that the compressed result is below a predetermined code quantity.
LITERATURE (1): "Activity Detection Method for Solid-State Still Video Camera Using DCT Data Compression" by Takizawa et. al., IEICE (Institute of Electronics.Information and Communication Engineers) Spring Conference, D-97, 1989, 7-79.
LITERATURE (2) "A Rate-Adaptive DCT Coding for use in Solid-State Still Camera" by Watanabe et. al., IEICE Spring Conference, D-159, 1989, 7-14.
The coding systems described in the above literatures are applicable only for the image that is obtained by a solid-state electronic camera with a small image pickup area. The number of pixels of the image is relatively small, e.g., 720.times.480 pixels. Accordingly, it is possible to start the image data processing after the image data is completely stored in the memory.
In the case of a full-color digital copying machine, the size of the image per se is large, and image data of three
colors is required. Therefore, the number of pixels to be processed is considerably large. For example, in an image of which the size is A3 (297 mm.times.429 mm), and the resolution is 400 spi (spot per inch), the number of pixels to be processed is 6720.times.4752 pixels. To store such a great number of pixel data items, the memory capacity must be large, and much time is required to process the image data.
In the circumstances, other coding parameter estimation methods operable at high speed and not requiring a large memory capacity are required.
One of the estimation methods is disclosed in Japanese Patent Unexamined Publication No. Sho. 63-222591. In the method disclosed, the data compressing method is based on the vector quantizing process. In the full-color digital copying machine, however, the DCT coding system is employed, as a standard coding system, for the data compressing system which is used for memory saving purposes. The data which is coded by the vector quantizing method is incompatible with the data which is coded by the DCT coding system, because the data coding systems are different from each other. To secure the communication of it with other devices, the coded data must be changed so as to be compatible with the coding systems of the other devices.